An inorganic electroluminescence device (hereinafter sometimes referred to as “inorganic EL device”) using an inorganic fluorescent substance as a light emitting material has been used, for example, in a display device such as a sheet-shaped light source as a backlight or a flat panel display. However, an inorganic electroluminescence device needs a high voltage alternating current for light emitting.
In recent years, an organic electroluminescence device (hereinafter sometimes referred to as “organic EL device”) has been reported which has a two-layered structure in which an organic fluorescent dye as a light emitting layer and an organic charge transport compound that is used in a photosensitizer of electrophotography are laminated on one another (JP-A-59-194393). A lot of attempts to use the organic EL device for device structures, organic fluorescent dyes and organic charge transport compounds have been reported because the organic EL device has the feature that it can easily provide light emissions of many colors in addition to the features of low voltage drive and high luminance in comparison with the inorganic EL device (Jpn. J. Appl. Phys., Vol. 27, page L269 (1988)), (J. Appl. Phys., Vol. 65, page 3610 (1989)).
Further, apart from the organic EL device that mainly uses a low molecular weight organic compound, WO9013148, JP-A-3-244630, Appl. Phys. Lett., Vol. 58, page 1982 (1991) or the like propose a polymer LED that uses a high molecular weight light emitting material. WO9013148 describes that a poly(p-phenylene vinylene) (hereinafter sometimes referred to as “PPV”) thin film that can be obtained by forming a film of a soluble precursor on an electrode followed by heat treatment to convert it into a conjugated polymer and a device using the same.
Moreover, JP-A-3-244630 describes a conjugated polymer characterized in that the polymer itself is soluble in a solvent and requires no heat treatment. Further, Appl. Phys. Lett., Vol. 58, page 1982 (1991) describes a polymer light emitting material soluble in a solvent and a polymer LED produced by using the same.
For the polymer LED, a film of an organic layer can be easily formed by coating. Therefore, in comparison with the case of depositing a low molecular weight fluorescent substance, the polymer LED can be advantageously produced in a large size with a low cost. Since the polymer LED is a polymer, it is considered excellent in mechanical strength of a film.
As the polymeric fluorescent substance to be used in these polymer LED, besides the above poly(p-phenylene vinylene), polyfluorene (Jpn. J. Appl. Phys., Vol. 30, page L1941 (1991), a polyparaphenylene derivative (Adv. Mater., Vol. 4, page 36 (1992) have been heretofore reported.
Thus, a method for producing a polymeric fluorescent substance that can provide a polymer LED with longer life has been demanded.